I. Field of the Invention
The present invention relates to communication systems, particularly indoor communication systems including cellular telephones, personal communication services (PCS), wireless private branch exchange (PBX) and wireless local loop telephone systems. More specifically, the present invention relates to a novel and improved distributed coaxial antenna for microcellular communication systems to facilitate indoor communications using spread spectrum signals.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as frequency hopping spread spectrum, time division multiple access (TDMA), frequency division multiple access (FDMA), and amplitude modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. However the spread spectrum modulation technique of CDMA has significant advantages over the other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, which is herein incorporated by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations (also referred to as cell-sites stations, cell-sites, or for short, cells) using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques thus permitting an increase in system user capacity.
The terrestrial channel experiences signal fading that is characterized by Rayleigh fading. The Rayleigh fading characteristic in the terrestrial channel signal is caused by the signal being reflected from many different features of the physical environment. As a result, a signal arrives at a mobile unit receiver from many directions with different transmission delays. At the UHF frequency bands usually employed for mobile radio communications, including those of cellular mobile telephone systems, significant phase differences in signals traveling on different paths may occur. The possibility for destructive summation of the signals may result in occasional deep fades.
Terrestrial channel fading is a very strong function of the physical environment of the mobile unit. A small change in position of the mobile unit or in the environment may change the physical delays of all the signal propagation paths, which further results in a different phase for each path. Thus for example, the motion of the mobile unit through the environment can result in a quite rapid fading process. For example, in the 850 MHz cellular radio frequency band, fading can typically occur as fast as one fade per second per mile per hour of vehicle speed. Fading this severe can be extremely disruptive to signals in the terrestrial channel resulting in poor communication quality. Additional transmitter power can be used to overcome the problem of fading. However, such an increase in power adversely effects both the user by excessive power consumption and the system by increased interference.
In a CDMA communication system, the same wideband frequency channel can be used for communication by all base stations. Typically in a FDMA scheme one frequency band is assigned to only one communication link, e.g. from the base station to one mobile unit. However in a CDMA system, the CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. Furthermore the high speed pseudorandom noise (PN) modulation allows many different propagation paths of a common signal to be separately demodulated at the receiving unit, provided the difference in path delays exceeds the PN chip duration, i.e. 1/bandwidth. If a PN chip rate of approximately 1 MHz is employed in a CDMA system, the full spread spectrum processing gain, equal to the ratio of the spreading bandwidth to system data rate, can be employed to discriminate against paths that differ by more than one microsecond in path delay from each other. A one microsecond path delay differential corresponds to differential path distance of approximately 1,000 feet. The urban environment typically provides differential path delays in excess of one microsecond, and up to 10-20 microseconds are reported in some areas.
In narrow band modulation systems such as the analog FM modulation employed by conventional telephone systems, the existence of multiple paths results in severe multipath fading. The only solution to fading in an FM system is to increase the transmission power. With wideband CDMA modulation, however, the different paths may be discriminated against in the demodulation process. This discrimination can be used to greatly reduce the severity of multipath fading.
It is desirable in the such communication systems that some form of diversity be provided which would permit a system to further reduce the effects of fading. Diversity is one approach for mitigating the deleterious effects of fading. Three major types of diversity exist: time diversity, frequency diversity, and space diversity.
Time diversity can best be obtained by the use of repetition, time interleaving, and error detection and correction coding which is a form of repetition. The present invention may employ each of these techniques as a form of time diversity. Because CDMA is inherently wideband, CDMA offers a form of frequency diversity because the signal energy is spread over a wide bandwidth. Therefore, frequency selective fading may affect only a small part of the CDMA signal bandwidth.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links between a mobile unit and two or more base stations. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention.
The deleterious effects of fading can be further controlled to a certain extent by controlling transmitter power. A fade which decreases the power received by the base station from the mobile unit can be compensated for by increasing the power transmitted by the mobile unit. The power control function operates in accordance with a time constant. Depending on the time constant of the power control loop and the duration of a fade, the system may be able to compensate for the fade by increasing transmit power of the mobile unit. A system for base station and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", also assigned to the assignee of the present invention.
The existence of a plurality of spatially different paths can provide space diversity to a wideband CDMA system. If two or more spatially different paths (such as provided by two spatially separated antennas) are available with differential path delay greater than one chip duration, two or more demodulation elements within a common receiver can be employed to separately demodulate the signals at a single base station or mobile unit. Because these signals typically exhibit independence in multipath fading, i.e., they usually do not fade together, the outputs of the two demodulation elements can be diversity combined in order to mitigate the adverse effects of fading. Therefore a loss in performance only occurs when both paths experience fades at the same time. Hence, one aspect of the present invention is the provision of two or more demodulation elements in combination with a diversity combiner.
In order to use multiple demodulation elements, it is necessary to utilize a waveform that is not only orthogonal to other signals in the system but is orthogonal with a delayed version of the same signal. A method and system for constructing PN sequences that provide orthogonality between the users so that mutual interference is reduced is disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", also assigned to the assignee of the present invention. Using these techniques in reducing mutual interference allows higher system user capacity and better link performance. With orthogonal PN codes, the cross-correlation between the codes is zero over a predetermined time interval, resulting in no interference between the orthogonal codes.
In cellular telephone systems, a large geographic area is provided with mobile telephone service by installing a number of base stations, each positioned to provide service to a corresponding limited base station coverage area. If service demands are great, the base stations may be subdivided or sectorized into smaller coverage areas or more base stations may be added. For example, some major metropolitan areas currently have nearly 400 base stations.
In a further development of a cellular telephone system, it may be desired to provide a number of very small base stations, called microcells, which would provide coverage of a very limited geographic area. Usually, it is considered that such areas are limited to a single floor of an office building and the mobile telephone service can be viewed as a cordless telephone system that may or may not be compatible with the external mobile cellular telephone system. The rationale for providing such a service is similar to the reasoning for use of Private Branch Exchange (PBX) systems in business offices. Such systems provide for low cost phone service for a large number of calls between phones within the business while providing simplified dialing for internal phone numbers. A few lines are also provided to connect the PBX system to the public telephone system, allowing calls to be made and received between telephones in the PBX system and telephones located elsewhere. It is desirable for the microcell system to provide a similar level of service but with the added feature of cordless operation anywhere within the service area of the PBX.
In the indoor environment and other areas bound by large surfaces in close proximity, path delays are typically much shorter in duration than experienced in the outdoor communication system environment. In buildings and other indoor environments where communication systems are used, it may be necessary to provide an additional form of diversity which enables discrimination between multipath signals.
A communication system adapted for indoor environments is described in the above-referenced U.S. Pat. No. 5,280,472 (the '472 patent), which is assigned to the assignee of the present invention, and which is herein incorporated by reference. Among other things, the '472 patent describes an implementation of a distributed antenna system which utilizes a single or dual set of discrete antennas where each discrete antenna on a common strand is separated from its neighbor with a delay element.
Also there are other less confined environments where it is desirable to have a coverage area shape different from the standard circular or cone shape provide by standard base stations. Even a serially connected set of discrete antennas which make up a distributed antenna provides less than ideal coverage over some linearly shaped regions. For example, a busy highway is a high capacity demand area. If discrete antennas are provided along the highway, the signal level must be large next to the antennas to reach the areas between the antennas. The large signal level may cause harmful intermodulation problems close to the base station while providing inadequate signal levels at the boundaries of the coverage area. Another even more problematic example is a subway or highway tunnel. A tunnel provides a unique environment in that the propagation paths are greatly confined. The confined paths result in strong and durationally short multipath propagation paths which result in relatively fast, flat, broadband fading. The fast rate of fading prevents power control from effectively compensating if the time constant of the power control is slower than the rate of the fades. Also the inherent broadband nature of the fast fades prevents the frequency diversity of the broadband CDMA waveform from mitigating the effects of the fast rate fading.
In such environments, it is more desirable to have an antenna system which provides an elongated, constant signal strength coverage area. If a distributed antenna comprised of a set of discrete antennas is thought of as having an antenna pattern resembling the light pattern from a string of Christmas tree lights, a more ideal antenna pattern would be one which has a coverage area similar to that of a neon tube light. The ideal antenna structure would also provide some form of diversity which would survive even the most harsh environment such as the tunnel environment. The present invention provides both a uniform coverage area and a reliable form of diversity.
In the above-referenced copending U.S. patent application Ser. No. 08/112,392, which is also assigned to the assignee of the present invention and incorporated herein by reference, a technique is disclosed for improving performance of a distributed antenna system using parallel strings of discrete antennas, each antenna on a common string is separated from its neighbors by a delay element. Two antennas one from each parallel string are placed at each node to provide spatial diversity throughout the coverage area. Thus the mobile unit in general has a similar distance, and hence path loss, to pairs of collocated antennas. The discrete antenna elements may include frequency conversion circuitry, thus reducing the cabling path loss between the antenna elements and the base station and allowing the use of readily available SAW devices as delay elements. At each discrete antenna node, circuits may be used to provide gain and duplexing functions.
Unfortunately, the circuitry associated with each antenna node can be relatively expensive and may require DC power to operate. Any cabling path loss occurring between nodes further increases DC power requirements, particularly for distributed antennas of appreciable length. Moreover, the accumulated delay associated with the SAW devices distributed along such lengthy systems may complicate efforts to achieve compliance with accepted telecommunications industry standards (e.g., IS-95).
It is a principal object of the present invention to provide a simple antenna system characterized by high capacity, simple installation, good coverage, and low susceptibility to multipath fading. The antenna system of the present invention advantageously provides these features without requiring DC power, and facilitates compliance with industry standards by creating less accumulated delay for a given length of antenna.